The present invention relates generally to printheads for ink-jet printers and, more particularly, to techniques for routing power and ground lines in an ink-jet printhead.
The art of ink-jet printing is relatively well developed. Commercial products such as computer printers, graphics plotters, and facsimile machines have been implemented with ink-jet technology for producing printed media.
Generally an ink-jet image is formed when a precise pattern of dots is ejected from a printhead onto a printing medium. Typically, an ink-jet printhead is supported on a movable cartridge that traverses over the surface of the print medium and is controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to a pattern of pixels of the image being printed. Typically, the cartridge includes the printhead and an ink reservoir.
A typical Hewlett-Packard ink-jet printhead includes an array of precisely formed nozzles in an orifice plate that is attached to a thin film substrate that implements ink firing heater resistors and apparatus for enabling the resistors. The thin film substrate is generally comprised of several thin layers of insulating, conducting or semiconductor material that are deposited successively on a supporting substrate in precise patterns to form, collectively, all or part of an integrated circuit. Deposition can be performed by mechanical, chemical or by vacuum evaporation methods.
An example of the physical arrangement of the orifice plate, ink barrier layer, and thin film substrate is illustrated at page 44 of the Hewlett-Packard Journal of February 1994. Further examples of ink-jet printheads are set forth in commonly assigned U.S. Pat. No. 4,719,477 and U.S. Pat. No. 5,317,346, both of which are incorporated herein by reference.
In a conventional ink-jet print cartridge, the printhead is formed using Tape Automated Bonding (TAB) and the printhead includes a nozzle member comprising two parallel columns of offset orifices formed in a flexible polymer tape by, for example, laser ablation. The tape is commercially available as, for example, Kapton.TM. from 3M Corporation. Other suitable tape may be formed of Upilex.TM. or its equilivalent. A back surface of the tape (i.e. the surface opposite the surface facing the recording medium) includes conductive traces formed thereon by a conventional photolithographic etching or plating process. The conductive traces are terminated by large contact pads designed to interconnect with a printer. In general, the print cartridge is installed on a printer so that the contact pads, on the front surface of the tape, contact printer electrodes providing externally generated electrical signals to the printhead.
Since the traces are formed on the back surface of the tape, access to them from the front of the tape is provided by vias formed through the front surface of the tape to expose the ends of the traces. These exposed ends are plated, with gold for example, to form the contact pads on the tape front. Typically, windows extending through the tape are used to facilitate bonding of the other ends of the conductive traces to electrodes on a silicon substrate containing heater resistors. The windows are filled with an encapsulant to protect underlying traces and substrate.
In the printhead, an ink barrier layer defining ink channels, is disposed between the thin film substrate and the orifice plate. Ink drop generator regions are formed by the ink chambers and portions of the thin film substrate and of the orifice plate that are adjacent the ink chambers.
The thin film substrate is typically comprised of compositions such as silicon nitride (S.sub.3 N.sub.4) and silicon carbide (SiC) on which are formed various thin film layers that form thin film ink firing resistors, apparatus for enabling the resistors, and interconnections to the bonding pads. In this regard, thin film topography can have a significant impact on printhead function. A typical thin film stack includes a plurality of thin film layers in which, for example, a silicon passivation layer is formed over a metallization layer and a tantalum passivation layer is deposited over the silicon layer. Finally, a gold layer is formed over the tantalum layer whereby the gold comprises the conductive traces and the bond pads. The gold layer is bonded to the ink barrier layer.
Generally, gold does not adhere well to other materials. With respect to the ink barrier layer, delamination between barrier material and gold is a concern. Such delamination can result in ink shorts, defined as electrical shorting, dendrite growth and electrochemical corrosion. This is especially the case near the tab bond window which comprises the interface between the bond pads and the thin film substrate and where ground, power and data lines on the flexible TAB circuit are bonded to the thin film substrate. Reliance on the encapsulant and adhesive techniques has not eliminated the delamination problem.
The problem of ink shorts in regions of delamination is exacerbated by the use of some newer inks. These contain organic solvents that can degrade adhesion between the ink barrier layer and the gold layer.
In view of the foregoing, it is apparent that a need exists for an ink-jet printhead having improved lamination quality combined with compatability with the newer corrosive inks.